PROJECT SUMMARY There is a fundamental gap in understanding how nitrogen metabolism is regulated in fungi. Nitrogen metabolism and its transcription regulators have been implicated in virulence of human fungal pathogens. Activation of nitrogen metabolism genes by the GATA transcription factor AreA is inhibited by the corepressor NmrA, but the mechanism of repression is not understood. Continued existence of this gap is an important problem because human fungal pathogens are a significant health and lifespan threat for immunocompromised individuals, and mechanisms controlling nutrient acquisition and metabolism by fungi is poorly understood. This research will narrow that gap through our long-term goal to understand gene regulatory mechanisms controlling transcription factor activity and metabolic gene expression in filamentous fungi. The overall objective of this proposal is to determine the role of a new gene AN4210 in nitrogen metabolite repression in the leading model for nitrogen regulation in filamentous fungi, Aspergillus nidulans. The central hypothesis is that this new gene, encoding a Mediator transcription complex component, is required for repression by the NmrA corepressor. This hypothesis was formulated based on preliminary data generated in the applicant's laboratory that identified the new gene as a suppressor of NmrA overexpression. The rationale for the proposed research is that understanding the mechanism of NmrA function, which is associated with virulence of human fungal pathogens, may lay foundations for development of novel treatments for fungal diseases. This hypothesis will be addressed by pursuing two specific aims: (1) Determine the role of the AN4210 Mediator complex gene in NmrA-mediated repression; and (2) Determine the physiological role of AN4210 in nitrogen regulation. The approach is innovative because it will exploit a novel mutant and a new gene we have identified that links the nitrogen transcription factors to the Mediator transcription complex in the Aspergillus nidulans nitrogen regulation model relevant for fungal pathogens, and because it represents a substantial departure from current thinking on the mechanism of NmrA-mediated repression thought to occur simply by interaction of NmrA with AreA. This proposed research is significant because it will determine how the NmrA corepressor mediates repression of the transcriptional activity of the Aspergillus nidulans transcription activator of nitrogen metabolic genes AreA, and the role of AN4210 in NmrA-mediated repression and in nitrogen regulation. Ultimately, understanding the molecular mechanism of NmrA action in the model may lead to treatments that prevent nutrient utilization by and growth of human fungal pathogens.